This application is the national phase under 35 U.S.C. xc2xa7 371 of PCT International Application No. PCT/JP00/00302, which has an International filing date of Jan. 21, 2000, which designated the United States of America and was not published in English.
1. Field of the Invention
The present invention relates to a polyamide ester. More particularly, the present invention is concerned with a polyamide ester which comprises a plurality of specific amide ester recurring units including, in a specific ratio, recurring units containing a tetravalent benzene group and recurring units containing a tetravalent diphenyl ether group, and which is adapted to be converted to a polyimide in a coating form by heat-curing, wherein the polyimide coating exhibits a residual stress of 33 MPa or less as measured with respect to a 10 xcexcm-thick polyimide coating formed on a silicon substrate. By the use of the polyamide ester of the present invention, it has become possible to prepare a photosensitive composition having excellent storage stability, wherein, for example, the change in viscosity of the composition during the storage thereof is suppressed. Further, the use of such a photosensitive composition enables not only the formation of a polyimide coating which simultaneously exhibits a low residual stress, a high chemical resistance, a high heat resistance and a high adhesiveness to a substrate, but also the formation of a polyimide pattern with a high resolution and a high precision. Therefore, the polyamide ester of the present invention can be advantageously used in the production of electric or electronic parts, such as a semiconductor device and a multilayer circuit board. Further, the present invention is also concerned with a polyamide ester composition having the same excellent effects as mentioned above in connection with the polyamide ester of the present invention.
2. Prior Art
Polyimide resins have excellent thermal and chemical stabilities, a low dielectric constant and a planation or planarization ability. Therefore, polyimide resins have been drawing attention as materials for use in fields related to microelectronics and, in fact, have been widely used as a material for a surface-protective film for a semiconductor, a material for an insulator layer used in a multilayer circuit structure, a material for a multichip module, and the like.
Generally, in the process for forming a polyimide resin coating in a desired pattern on a semiconductor device, a polyimide resin coating is first formed on a substrate and then, a desired pattern is formed on the polyimide resin coating by a lithographic technique. In this case, the desired pattern is formed through an indirect procedure. Specifically, a polyimide resin pattern is formed by a process in which a photoresist pattern (corresponding to a desired polyimide resin pattern) is formed on a polyimide resin coating using a photoresist and a photomask, followed by etching to obtain a polyimide resin pattern. This process has problems in that it requires complicated operations (wherein a photoresist pattern serving as a mask for a subsequent etching operation is first formed on a polyimide resin coating and, then, the polyimide resin is etched, and, finally, the photoresist pattern, which no longer is necessary, is removed), and that, since the desired pattern is formed through an indirect procedure, the resolution is low. In addition, this process has a problem of safety, since this process requires the use of a poisonous substance, such as hydrazine, as a solvent for etching.
In recent years, for solving the above-mentioned problems, studies have been made on a method using a polyimide precursor containing a photopolymerizable photosensitive group. In this method a desired pattern is directly formed in a polyimide precursor coating. For example, there has been proposed a method which comprises: forming on a substrate a coating of a photosensitive composition (comprising a polyimide precursor composed of a polyamide acid derivative having bonded thereto a double bond-containing compound through an ester linkage, an amide linkage, an ionic linkage or the like, and a photoinitiator); exposing the coating through a photomask (having an image corresponding to a desired pattern), so that the polyimide precursor present in the exposed area of the coating becomes insoluble to a developer, thereby forming a latent pattern image in the coating; subjecting the resultant coating to a developing treatment, thereby obtaining a desired pattern of the polyimide precursor; and heating the obtained pattern to remove photosensitive group-containing components (such as the above-mentioned double bond-containing compound and the photoinitiator), thereby converting the polyimide precursor to a polyimide having heat stability (Yamaoka and Omote, xe2x80x9cPori fairu (Polyfile)xe2x80x9d, Vol. 27, No. 2, pages 14 to 18, 1990). This method is generally called a photosensitive polyimide technique. By this technique, the above-mentioned problems accompanying the conventional processes using a non-photosensitive polyimide have been overcome. Therefore, the above-mentioned photosensitive polyimide technique has been put into wide use in the formation of a polyimide pattern.
However, in recent years, demands for higher resolution in the formation of a polyimide pattern used in a semiconductor device and the like have increased. In the processes using a non-photosensitive polyimide (which were developed prior to the development of the above-mentioned photosensitive polyimide technique), the resolution is low, and such low resolution is taken into consideration when the semiconductor devices per se and the production processes for semiconductor devices are designed. Therefore, the degree of circuit integration of the semiconductor devices and the precision of the semiconductor devices are necessarily low. On the other hand, by the use of the photosensitive polyimide, it is possible to achieve a high resolution in the formation of a polyimide pattern and, hence, semiconductor devices having a high degree of circuit integration and a high precision can be produced. On this point, an explanation is made below. For example, in the production of a memory device etc., in order to increase productivity, commonly employable circuits are first formed and, after examination of the memory device etc., unnecessary circuit portions are cut-off. In conventional processes using a non-photosensitive polyimide, this cutting step is conducted before forming the polyimide patterns. On the other hand, in the processes using a photosensitive polyimide, it has become possible to obtain high resolution patterning of the polyimide. Therefore, in forming polyimide patterns on circuits, appropriate holes are first formed in the polyimide pattern film, so that cutting of unnecessary circuit portions can be performed through the holes after the formation of the polyimide pattern film. The cutting-off of unnecessary circuit portions after forming polyimide patterns enables the yield of products to be improved, since the timing of the cutting-off of the unnecessary circuit portions is close to the completion of the final products, as compared to the case of the conventional processes using a non-photosensitive polyimide.
In the above-mentioned process in which the cutting of unnecessary circuit portions is performed through the holes after the formation of polyimide patterns, it is desired that the holes be as small as possible so as to achieve a high degree of circuit integration of the device. Accordingly, the demand for photosensitive polyimide precursors capable of forming a pattern with an improved resolution has increased. When such a photosensitive polyimide precursor is used, it becomes possible to achieve a wide process margin which is necessary for efficiently producing a semiconductor device having a high degree of circuit integration and a high precision. (A process having a xe2x80x9cwide process marginxe2x80x9d means a process in which the employable ranges of conditions (e.g., time and temperature) for the exposure and the development in the formation of a pattern are wide.) Therefore, the higher the resolution in the formation of a polyimide pattern, the more desirable the photosensitive polyimide precursor. This applies to the case of polyimide patterns used in other devices, such as a multichip module. Therefore, there has been an increasing demand for a photosensitive polyimide precursor composition capable of forming a polyimide pattern with a high resolution and a high precision.
Since the required polyimide coatings tend to be thick and the required printed circuits tend to be more dense, the demand for high resolution photosensitive polyimide precursor compositions is high.
Further, when a polyimide coating has a poor heat resistance, such a coating is deteriorated during the heat-curing in the formation of a pattern. For this reason, the heat resistance of a polyimide film is also important.
In recent years, there has been a marked trend to increase the diameter of a silicon wafer (substrate) used in the production of a semiconductor device so as to improve the production efficiency. When a polyimide is used for forming a surface protective-coating for a semiconductor device, a stress is generated at the interface between the substrate and the polyimide coating due to the difference in the degree of shrinkage (caused when a polyimide coating formed on a substrate by heat-curing is cooled to room temperature) between the substrate and the polyimide coating. This stress causes warpage of the substrate. The larger the diameter of the substrate, the larger the warpage. When the warpage of the substrate is large, disadvantages are likely to be caused such that efficiency of the production process is adversely affected and that the cracking of the polyimide coating occurs. Therefore, it has been desired to develop a polyimide precursor capable of forming a polyimide coating having a satisfactorily small residual stress.
Further, when it is intended to use a polyimide to form a surface protective coating for a semiconductor device, it is required that the polyimide coating exhibit a high adhesiveness to a substrate, and that the adhesion of the polyimide coating to the substrate has a high water resistance (these required properties are hereinafter, collectively referred to as xe2x80x9cwater resistant adhesionxe2x80x9d).
Further, a polyimide coating is expected to have high mechanical properties, especially, a high elongation. The production process for semiconductor devices involves a step in which a polyimide coating is exposed to a high temperature (400xc2x0 C. or more) and hence, a polyimide coating is required to have a high heat resistance such that the change in properties of the coating, such as mechanical properties (e.g., elongation), can be prevented even under high temperature conditions. The polyimide coating is also required to have a high chemical resistance.
It has been desired to develop a photosensitive polyimide precursor which can be used for forming a polyimide pattern having all of the above-mentioned required properties. However, as explained below, such an excellent photosensitive polyimide precursor has conventionally not been realized.
Conventionally, a tetracarboxylic acid dianhydride and a diamine are usually used as monomers for producing a polyimide. Since a homopolyimide having all of the above-mentioned required properties has not been obtained, a number of proposals have been made on various combinations of monomers for synthesizing an excellent polyimide. For example, with respect to the non-photosensitive polyimide and the non-photosensitive polyimide precursor, Unexamined Japanese Patent Application Laid-Open Specification No. 60-147441 (corresponding to U.S. Pat. No. 4,590,258 and EP154720B1) describes a technique for producing a polyamide acid, in which pyromellitic acid anhydride (PMDA), and oxydiphthalic acid anhydride (ODPA) or benzophenone tetracarboxylic acid are used as tetracarboxylic acid dianhydrides, and these tetracarboxylic acid dianhydrides are reacted with oxydianiline. Further, this prior art document also describes a polyimide obtained by imidizing the above-mentioned polyamide acid. However, the polyimide obtained from ODPA has a problem in heat resistance. It is known that the heat resistance of a polyimide in a copolymer form obtained by copolymerizing ODPA and PMDA is lowered almost linearly as the content of ODPA in the obtained polyimide is increased. This is apparent from the data of the weight loss test conducted at 500xc2x0 C. for 1 hour, which are shown in Table 2 of the above prior art document.
As an example of the photosensitive polyimide technique, there can be mentioned methods in which a photosensitive polyimide precursor is obtained through a polyamide acid. Specific examples of such methods include a method disclosed in Examined Japanese Patent Application Publication No. 59-52822 (corresponding to U.S. Pat. No. 4,243,743), in which a photosensitive polyamide acid is obtained by introducing a photosensitive group to a polyamide acid through an ionic linkage, and a method disclosed in Examined Japanese Patent Application Publication No. 4-623062 (corresponding to U.S. Pat. No. 4,551,522 and EP203372B1), in which a polyamide acid is converted to a polyisoimide, and the obtained polyisoimide is reacted with an alcohol to obtain a photosensitive polyamide ester. However, as in the case of the above-mentioned non-photosensitive polyimide, a photosensitive homopolymer form of a polyamide acid or polyamide ester cannot be used as a photosensitive polyimide precursor for obtaining a polyimide having all of the above-mentioned required properties. Further, as in the case of the above-mentioned polyamide acid in a copolymer form obtained by copolymerizing ODPA and PMDA, even when a copolymer obtained by introducing a photosensitive group to a copolymer of a plurality of different tetracarboxylic acid dianhydrides is used as a polyimide precursor, it is impossible to produce a polyimide pattern having all of the above-mentioned required properties. This is because all of the different tetracarboxylic acid dianhydrides have their respective different defects, and a defect of a certain tetracarboxylic acid dianhydride cannot be compensated by the use of other tetracarboxylic acid dianhydrides. Specifically, a polyimide produced from ODPA has problems, such as a low heat resistance which leads to a lowering of the pattern precision caused during the heat-curing, a low chemical resistance, and a high residual stress. On the other hand, a polyimide produced from PMDA has problems different from those of the polyimide obtained from ODPA, i.e., a low water resistant adhesiveness, a low elongation and the like. Conventionally, there has been no polyimide which is free from all of these problems.
Unexamined Japanese Patent Application Laid-Open Specification No. 2-135274 (corresponding to U.S. Pat. No. 4,954,578 and EP366307B1) proposes a method in which a mixture of solutions of two different polyimides which are incompatible with each other is used to form a polyimide coating having a microstructure in which the microdomains of the two different polyimides are present and the size of each microdomain is 1 xcexcm or less. In this method, such a microstructure is formed in an attempt to obtain a polyimide coating having the respective properties of both of the two different polyimides. However, when this method is applied to the photosensitive polyimide technique, the following problem arises. When a mixture of solutions of two different photosensitive polyimide precursors which are incompatible with each other is used to form a photosensitive polyimide precursor coating, such a photosensitive polyimide precursor coating contains respective domains of the polyimide precursors, wherein each domain has a size which is a little smaller than or larger than the wavelength of light. Therefore, light is diffused in the coating, leading to a lowering of the pattern precision and the resolution.
In this situation, the present inventors have made extensive and intensive studies with a view toward developing a photosensitive polyimide precursor which can be advantageously used for forming a polyimide pattern which simultaneously exhibits a low residual stress, a high water resistant adhesion, a high elongation, a high chemical resistance, a high heat resistance and a high precision. As a result, it has unexpectedly been found that a specific polyamide ester obtained from tetracarboxylic acid dianhydrides including a tetravalent diphenyl ether group-containing tetracarboxylic acid dianhydride (such as ODPA) and a tetravalent benzene group-containing tetracarboxylic acid dianhydride (such as PMDA) exhibits only the respective advantageous effects of the above two different tetracarboxylic acid dianhydrides, and such a specific polyamide ester can be used as an excellent photosensitive polyimide precursor. The above-mentioned specific polyamide ester comprises recurring units derived from a tetravalent diphenyl ether group-containing tetracarboxylic acid dianhydride (such as ODPA) (which, when used in conventional processes for producing a polyimide, has an effect of improving the water resistant adhesion and the elongation of a polyimide but has a defect of lowering the heat resistance of a polyimide), and recurring units derived from a tetravalent benzene group-containing tetracarboxylic acid dianhydride (such as PMDA) (which, when used in conventional processes for producing a polyimide, has an effect of improving the heat resistance and the chemical resistance of a polyimide and an advantageous effect of lowering the coefficient of thermal expansion but has a defect of lowering the water resistant adhesion and the elongation of a polyimide) in specific molar ratios, relative to the total molar amount of the recurring units, which polyamide ester is adapted to be converted to a polyimide in a coating form by heat-curing, wherein the polyimide coating exhibits a residual stress as low as 33 MPa or less as measured with respect to a 10 xcexcm-thick polyimide coating formed on a silicon substrate. Further, it has also been found that a polyamide ester composition comprising a plurality of different polyamide esters, wherein the plurality of different polyamide esters collectively contain recurring units derived from the above-mentioned tetravalent diphenyl ether group-containing tetracarboxylic acid dianhydride and recurring units derived from the above-mentioned tetravalent benzene group-containing tetracarboxylic acid dianhydride in specific molar ratios, relative to the total molar amount of the recurring units of the different polyamide esters, can also be used as a polyimide precursor having the same excellent effects as mentioned above in connection with the specific polyamide ester. The present invention has been completed, based on these novel findings.
Accordingly, it is a primary object of the present invention to provide a polyamide ester which can also be advantageously used for obtaining a photosensitive composition capable of forming a polyimide pattern which simultaneously exhibits a low residual stress, a high water resistant adhesion, a high elongation, a high chemical resistance, a high heat resistance and a high precision.
Another object of the present invention is to provide a polyamide ester composition which can be advantageously used for obtaining the above-mentioned excellent photosensitive composition.
Still a further object of the present invention is to provide a photosensitive composition having excellent effects as mentioned above.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and claims taken in connection with the accompanying drawing.